A Waikato man who bashed his partner and threw a car part at her, hitting their baby, because she hadn't parked close enough to a house has been sentenced to 2-1/2 years' jail.

Hamilton District Court Judge Glen Marshall told Willy-Joe Wilson, 27, he was "very lucky" he didn't kill his 1-year-old son after he threw a metal gear lever at his partner. Wilson missed and hit the boy in the forehead, leaving a large, bleeding gash.

Judge Marshall said it was also lucky the boy didn't have any brain damage.

The incident unravelled on January 30 when Wilson, his partner, and their three children drove to a relative's house in Waharoa.

Wilson, of Putaruru, became angry with his partner as he believed she had parked too far away from the house.

He became abusive, got his son out of his car seat and walked around to the driver's door. He put his son on the ground before punching his partner and dragging her out of the car.

People came out of the house to stop the attack.

Wilson went to the car and grabbed the metal gear lever before throwing it. He then fled with his two daughters, aged 5 and 6.

His son needed 10 stitches to his head, while his partner had bruises and cuts.

Wilson's lawyer, Mark Sturm, said it was "unquestionable" that the facts were disturbing and an over-reaction to a domestic dispute where "his conduct has got completely out of hand".

"He does bitterly regret what has occurred and the injury he has caused to his son," Sturm said.

"He does wish to emphasise that this was not deliberate ... he didn't intend for his son to be injured and didn't want him to be injured, but realises that the way he acted has caused it."

Sturm said Wilson's partner was keen to stay in the relationship with Wilson if he took steps to address his anger issues.